Hypocenter relocation of the aftershocks of the Mw 7.5 Palu earthquake (September 28, 2018) and swarm earthquakes of Mamasa, Sulawesi, Indonesia, using the BMKG network data

On September 28, 2018, the Mw 7.5 earthquake occurred in Palu, Central Sulawesi, Indonesia. This earthquake produced strong tremors, landslides, liquefaction and a tsunami and caused thousands of fatalities and damaged houses and infrastructure. We have relocated 386 of the 554 Palu aftershocks by using the double-difference relocation method (hypoDD) from September 28 to November 22, 2018. The aftershock pattern is consistent with the crustal deformation in the area and generally shows that the events have a NW–SE trending of ~ 200 km in length and ~ 50 km in width. Most of the aftershocks are located to the east of the Palu-Koro Fault Line. Since November 2, 2018, there have been hundreds of swarm earthquakes in the area of Mamasa, West Sulawesi, which is about 230 km south of the city of Palu. Some of these earthquakes were felt, and houses were even damaged. We have relocated 535 of the 556 swarm earthquakes having a magnitude of M 2 to M 5.4. Our results show that the seismicity pattern has a dip that becomes shallower to the west (dipping at a ~ 45° angle) and extends from north to south for a length of ~ 50 km. We also conducted a focal mechanism analysis to estimate the type of fault slip for selected events of an M > 4.5 magnitude. Most of the solutions of the focal mechanism analysis show a normal fault type. This swarm earthquake probably corresponds to the activity of the fault in the local area.

Generating Behaviors of Strong Tremors and Experimental Study of Rockburst-Triggering Criterion

Numerous tremors occur during excavation and mining periods in coal mines, and low-energy tremors often occur before strong tremors. In addition, it is found that not all strong tremors cause rockbursts. Therefore, generating behaviors of strong tremors and the triggering criterion for the occurrence of the rockburst must be investigated. In this study, generating behaviors of strong tremors were inferred by velocity tomograms and energy density clouds. An innovative facility capable of applying dynamic disturbance to roadway models was developed to study the triggering criterion for rockbursts. Velocity tomograms indicated that the stress concentration extended to the syncline region with the advance of the coalface. High-energy-density clouds expanded rapidly and regularly in some areas that were parts of the high-velocity region, indicating that the stress increased rapidly in these areas, until strong tremors took place nearby. AE activities suggested that the modelled roadway offered good resistance as the dynamic loading energy grew from 29.4 J to 117.6 J. Then, sharp AE activity at the dynamic loading energy of 147 J indicated the ultimate shock resistance of the roadway was almost reached. Finally, bursting failure of the modelled roadway was observed at a dynamic loading energy of 176.4 J.

Understanding Rockburst-Generating Behaviors and Associated Seismicity by Using a Spatial Calculation Methodology with an Energy Density Index

Rockbursts have become one of the most severe risks in underground coal mining. A proper understanding of the relationship between the spatial activities of mining-induced tremors and the occurrence of rockbursts can provide effective insight into the evaluation of rockburst hazard as well as revealing their causes. A methodology for spatially calculating the seismicity involving the use of an energy density index was developed to identify the evolution of mining-induced tremors over time. The results showed that numerous tremors occurred during the excavation and mining periods, and those tremors were distributed in a spatially complicated fashion, and it was difficult to identify their evolution trends over time and assess the rockburst hazard. However, energy density clouds had obvious distinguishable trends that presented nucleation characteristics and followed obvious extension around the nucleuses until strong tremors took place nearby. Velocity tomograms indicated that evolution of energy density clouds was the response to the rising stress concentration in some local areas before the rockburst. Then the rockburst-generating journey was inferred; that is, the jump of stress in local areas of coal-rock masses results in the clustering and nucleation of microfractures firstly, and then as the microfractures developed, macrofractures appeared, bringing strong tremors which triggered the rockburst.